Many liquids used in industrial processes contain entrained and/or dissolved gases such as air as a consequence of the mode of their preparation or purification. Removal of these gases is frequently necessary to carry out the aforementioned industrial processes satisfactorily, but this is often difficult to accomplish as, for example, in the case of foamy or viscous liquids. In the manufacture of photographic products, in particular, it is essential to remove bubbles of air from the prepared solutions, which can be foamy as well as viscous, and to prevent subsequent formation of new bubbles in the solutions in order to ensure a uniform, smooth coating of the required thickness.
U.S. Pat. No. 2,634,907 discloses a centrifuging process and apparatus for continuously removing gas in which the liquid to be deaerated is fed into the centrifugal zone of a cylindrical rotor and the deaerated liquid is removed through a liquid seal into a substantially gas-free collecting zone. The level of liquid within the rotor chamber is preferably controlled by the size of the opening in an orifice plate valve located at the outlet.
U.S. Pat. No. 2,785,765 discloses a centrifuging apparatus for degassing liquids, which apparatus contains, in an air-tight housing, a generally dome-shaped rotatable member under a condition of partial vacuum into which liquid is fed through a "buttering ring" and from which the treated liquid is removed by a scoop that is a hollow tube of curved contour.
U.S. Pat. No. 3,228,595 discloses a centrifuge discharge means that is a skimmer having an elongate body and a leading end that is divided into an upper opening and a lower opening. The skimmer in operation is only partly immersed in the liquid to reduce the foaming tendency of the liquid.
Some fluids which are required to be degassed are not solutions but rather suspensions or emulsions. U.S. Pat. No. 3,996,117 discloses a discharge device for removing a fluid from a fluid processing apparatus, which device causes the fluid to flow around its converging side surfaces in a streamlined flow pattern and causes sufficient sub-surface turbulence to keep particles originally dispersed throughout the fluid from settling toward the bottom, which device further, by means of one or more inlet ports in its side surfaces, enables fluid to flow to the outlet of the fluid processing apparatus.
A vacuum degassing centrifuge apparatus ideally operates in a steady state condition in which the level of degassing liquid in the annular trough of the centrifuge bowl is maintained at a substantially constant level. This condition could theoretically be attained by using two perfectly matched pumps that produce exactly equal inlet and outlet rates. In practice this situation is virtually impossible to achieve, particularly when one takes into account other potential disruptions in this theoretical equilibrium condition, for example, when it becomes necessary to change a filter in the system. Under these circumstances there is a need for an additional means of controlling the level of liquid in the centrifuge bowl.
One such means of liquid level control that has been employed utilizes sensing probes located within the trough of the centrifuge bowl. In particular, conductivity sensors have been mounted on the leading edge of the centrifuge discharge device disclosed in U.S. Pat. No. 3,966,117. If a conductivity sensor immersed in the collected liquid becomes uncovered, a valve is opened to allow more liquid to enter the bowl. As the sensor again becomes covered, the valve is closed to stop the inflow.
The above described method of sensing and controlling the level of liquid in the centrifuge bowl is disadvantageous in that the operation of the valve is abrupt, leading to significant pressure fluctuations. The larger variations in pressure give rise to a hydraulic hammering effect that damages filters in the system. Smaller pressure changes, on the other hand, cause flow spikes in the degassed liquid exiting the apparatus; if that liquid is, for example, a photographic emulsion to be coated, an uneven, defective coating is the result.